This article analyses the legal origins of the ‘substance of rights’ doctrine, and its judicial development since its creation in landmark Union citizenship cases over a decade ago. It is demonstrated how the status of Union citizenship has evolved from being a proclaimed fundamental status for the individual in a lawful cross-border situation, to an increasingly operational and legally effective status regardless of the nature of the free movement situation. Under a genuinely substantive status of Union citizenship, any and all Member States are obligated to neither restrict freedom of movement under art. 21 TFEU, nor deprive, de jure or de facto, a Union citizen of the genuine enjoyment of the substance of Union citizenship rights under art. 20 TFEU. Thereby, the relevance of art. 20 TFEU is no longer reserved to the Union citizen’s relationship to their home Member State. In addition, it is argued that, as the jurisdictional spheres of art. 21 TFEU and 20 TFEU merge, the legal mechanisms of EU fundamental rights protection should also be streamlined across Directive 2004/38, art. 21 TFEU and art. 20 TFEU; thereby giving further substance to the citizenship ideal of civis europaeus sum.